Such a hydraulic machine is, for example, known from DE 195 20 405 C2 and DE 195 20 402 C2. In such machines, the gear wheel orbits inside the gear ring. For this purpose, a tooth number difference exists between the gear wheel and the gear ring, said difference often being 1. The movement of the gear wheel is transferred to a drive shaft via an articulated shaft.
If the hydraulic machine is operated as a hydraulic motor, a working medium with a working pressure, for example a hydraulic fluid, is supplied at the inlet connection, whereas the outlet connection is usually connected to a tank or at least to a low pressure level. Via the commutation section that comprises a rotary slide valve and a valve plate the hydraulic fluid is guided into selected working chambers of the gear wheel in such a way that an orbiting movement is generated. Depending on the position of the gear wheel, the rotary slide valve and the valve plate always only pressurize the actually required working chambers, while the remaining working chambers are connected to the outlet connection via the rotary slide valve and the valve plate.
When using the hydraulic machine as pump, the drive shaft is driven from the outside. This causes that the gear wheel orbits inside the gear ring and expands and contracts the working chambers one after the other. Thus, a working medium is sucked in through the inlet connection and discharged through the outlet connection. The allocation of the individual working chambers to the inlet connection or the outlet connection that depends on the position of the gear wheel is predetermined by the valve plate and the rotary slide valve.
In order to keep leakages between the rotary slide valve and the valve plate small, it is known, for example from DE 195 20 405 C2, to press the rotary slide valve against the valve plate by means of a pressure plate. The pressure plate is acted upon by the pressure of the incoming working medium. Additionally, a spring is provided that presses the rotary slide valve against the valve plate, so that a sufficient tightness is also ensured when the inlet pressure is low. In order that the pressure plate can also supply a sufficient pressure against the rotary slide valve during a reversal of the rotation direction of the machine, during which also the pressures at the inlet connection and the outlet connection are reversed, a relatively complicated design of the pressure plate is required. This causes on the one side that a relatively large space is required, and on the other side the manufacturing of the machine is also rather cost intensive.